Industrial combustors of the diffusion combustion type are used widely for driving gas turbines for aircrafts and power generators. Air flows into the conventional combustor not only through the fuel nozzle and cooling air supply openings formed in a combustor liner, but also through a dilution air supply opening formed on the downstream side of the fuel nozzle. The fuel delivered by the diffusion fuel nozzle is diffused and mixed into part of the air flowed into the combustor, and burns in the combustor. The rest of the air that did not flow through the fuel nozzle and cooling air supply openings, flows through the dilution air supply opening formed in a downstream part of the combustor into the combustor and is used as air for combustion and for diluting a high-temperature gas to reduce the temperature of the gas to a temperature proper for an engine cycle. A fuel mixture prepared by mixing the fuel and air by the fuel nozzle is not uniform. The spatial distribution of the fuel concentration of the fuel mixture is irregular. The fuel spray angle of the fuel injection nozzle and the position of an igniter are determined so as to make a part of the flammable fuel mixture having a high fuel concentration flow near to an igniter so that the fuel mixture can be easily ignited. Thus the combustor is particularly excellent in ignition performance. Blow-off rarely occurs while the combustor is operating in a low-output operation mode and the combustor is excellent also in flame stability.
In the combustor of the diffusion combustion system, there is a high-temperature frame region because the spatial distribution of the fuel concentration of the fuel mixture is irregular. Consequently, NOx (nitrogen oxide) is produced at a high rate particularly while the combustor is operating in a high-output operation mode. A large amount of NOx emission is undesirable from the viewpoint of environmental protection and air pollution prevention. The exhaust gas of gas turbine engines need to meet the recent severe environmental criteria.
Flame temperature needs to be lowered to reduce NOx emission. To lower NOx emission, uniform mixing of fuel and air and use of a lean fuel mixture determined in connection with a desired level of NOx emission for combustion are necessary. A gas turbine of the lean premixed combustion system is one of measures to reduce NOx emission. In the lean premixed combustion system, a lean uniform fuel and air mixture is produced prior to combustion so as to achieve low NOx emission.
The lean premixed combustion system can achieve low NOx emission provided that air distribution in the combustor is designed so that the equivalence ratio of a fuel mixture prepared by the fuel nozzle for an operation in a maximum-output operation mode in which NOx emission increases to a maximum is low. The fuel mixture needs to be so lean that the equivalence ratio is on the order of 0.7 or below for an operation in a maximum-output operation mode. In this case the equivalence ratio of a fuel mixture to be supplied to the combustor of an aircraft gas turbine for a typical idling operation is between about 0.2 and about 0.3. Such a fuel mixture is excessively lean. Under such a condition, it is possible that the low temperature of compressed air supplied to the fuel nozzle and the combustor may cause poor combustion efficiency or blow off. Whereas uniform mixing of a large amount of air and fuel in the fuel nozzle for the lean premixed combustion system is advantageous in reducing NOx emission during operations in a maximum-output operation mode, the same makes combustion unstable during an operation in a low- or a middle-output operation mode, may possibly cause blow off and reduces combustion efficiency.
Fuel injection nozzles have been developed in recent years to solve problems in the diffusion combustion system and the premixed combustion system. A hybrid fuel injection nozzle is one of those recently developed fuel injection nozzles. The hybrid fuel injection nozzle has, in combination, a pilot fuel injection nozzle for diffusion combustion and a main fuel injection nozzle, for premixed combustion, coaxially surrounding the pilot injection nozzle. The central pilot fuel injection nozzle mixes fuel with a comparatively small amount of air to produce a rich fuel mixture. The outer main fuel injection nozzle mixes fuel with a comparatively large amount of air to produce a lean fuel mixture. The main fuel injection nozzle operates only during operations in a middle- and a high-output operation mode.
In a combustor provided with the hybrid fuel injection nozzle, only the pilot fuel injection nozzle are fuelled during an operation in a low-output operation mode and the fuel delivered by the pilot fuel injection nozzles is mixed with only air passed through the pilot fuel injection nozzles. Thus a comparatively rich fuel mixture is produced locally to improve flame stability and combustion efficiency during an operation in a low-output operation mode. A lean fuel mixture is produced by fuelling both the pilot and the main fuel injection nozzles during an operation in a high output operation mode to stabilize combustion and to reduce NOx emission. During an operation in a middle-output operation mode, the number of the working main fuel injection nozzles is changed according to the engine output conditions, namely, temperature and pressure at the inlet of the combustor) to use fuel mixtures of equivalence ratios in a proper range in which combustion efficiency is satisfactory and blow off may not occur. When the hybrid fuel injection nozzle is used, the flames can be stabilized in a wide range of engine operations, and NOx emission can be reduced during an operation in a high-output operation mode.
During an operation in a low-output operation mode, such as an ignition operation or a relight operation, the outer main fuel injection nozzle of the hybrid fuel injection nozzle are not used for fuel injection and air passed through the main fuel injection nozzle forms an air layer in the vicinity of the inside surface of a wall defining a combustion chamber. This air layer obstructs the flow of a fuel mixture produced by the pilot fuel injection nozzle to a region around the igniter. It may be possible for comparatively large fuel drops having high inertia force among fuel drops jetted out by the pilot fuel injection nozzle to penetrate the air layer and to reach the region around the igniter. However, the pilot fuel injection nozzle needs undesirably to inject a large amount of fuel to produce a combustible fuel mixture by mixing fuel into the main air layer. In principle, the pilot fuel injection nozzle is required to produce fine spray to ensure that the exhaust gas produced by combustion has a satisfactory property. Thus it is undesirable to jet large fuel drops capable of penetrating the air layer.
To ignite the fuel mixture delivered by the fuel injection nozzle into the combustion chamber, the igniter needs to discharge high-energy sparks capable of reaching the combustible fuel mixture delivered by the pilot fuel injection nozzle through the air layer formed in the combustion chamber. A combustor intended to solve such an ignition problem is provided with an igniter capable of being moved in a combustion chamber in diametrical directions. Such a combustor is proposed in Patent document 1.
This previously proposed combustor positions the igniter at a diametrically inner position in the combustion chamber during an operation in an ignition such that the tip of the igniter is positioned near the combustible fuel mixture delivered by the pilot fuel injection nozzle to improve ignition performance. After the completion of ignition, the igniter is retracted to a diametrically outer position in the combustion chamber to space the tip of the igniter apart from flames to prevent the breakage of the tip of the igniter and the disturbance of flows resulting from exposure of the igniter to flames.    Patent document 1: JP 2000-18051 A